TIME RESOLVED SPECTROSCOPY [T.R.S.]: Refresher Course in Chemistry: “Highlights of Structure and Reactivity for Today’s Chemist” TIME RESOLVED SPECTROSCOPY [T.R.S.]: Principles and Scope of the TRS technique for studying the Molecular Processes Dr. S. Aravamudhan Department of Chemistry North Eastern Hill University SHILLONG Thursday, September 19, 2008 April 15, 2019 Refresher Course

( time-dependent Infra red spectra by rapid scan) Transient-absorption spectroscopy (Rapid scan conventional CW) Pulsed transient decay techniques Time Resolved Spectroscopy: Fluorescence Spectra Transient-absorption spectroscopy ( time-dependent Infra red spectra by rapid scan) Transient-absorption spectroscopy is an extension of absorption spectroscopy. Here, the absorbance at a particular wavelength or range of wavelengths of a sample is measured as a function of time after excitation by a flash of light. In a typical experiment, both the light for excitation ('pump') and the light for measuring the absorbance ('probe') are generated by a pulsed laser. If the process under study is slow, then the time resolution can be obtained with a continuous (i.e., not pulsed) probe beam and repeated conventional spectrophotometric techniques. Time-resolved Infrared Spectroscopy of the Ca21-ATPase THE ENZYME AT WORK* April 15, 2019 Refresher Course

Transition state signal Flash Intensity of transition state signal B C A 0% 20% 32.5% 47.5% 67.5% 77.5% 100% Transition state signal v TSS The absorbance at this frequency v is only from product Product Flash Abs at V Reactant Growth plot April 15, 2019 Refresher Course t

This released ATP can be catalysed by Enzyme “ATPase” When Ca+2 binds, the enzyme uses the ATP as the substrate and ATP to ADP conversion takes place with the phosphorylation of the ENZYME A bilogically inactive precursor from which no ATP release takes place. When a light flash is released photochemically the ATP release takes place. Calcium release follows and the Enzyme in ADP sensitive form E1, is rendered as ADP insensitive form E2 This released ATP can be catalysed by Enzyme “ATPase” This enzyme is activated by Calcium binding. E1 to E2 Conversion results in conformational change & manifests in the differences of the backbone amide IR bands. One IR spectral run was of 65ms. (rapid scan) Bruker IFS66 resolution 4cm-1 April 15, 2019 Refresher Course

10 spectra of 40scan(2.6s), 10 Spectra of 4scan(0.26s), Light flash 10 spectra of 40scan(2.6s), 10 Spectra of 4scan(0.26s), 10spectra 1scan (0.065s), Time interval 0.26s 0.26s 10spectra 1scan (0.065s) 10 Spectra of 4scan(0.26s) 10 spectra of 40scan(2.6s) 10 spectra of 300scan(19.5s) 10spectra 1scan (0.065s), 10 Spectra of 4scan(0.26s), April 15, 2019 Refresher Course

April 15, 2019 Refresher Course

Generation and detection of terahertz pulses Time-resolved terahertz spectroscopy 1012 Hz Picosecond regime Molecular aggregates studied by time-resolved terahertz spectroscopy Time-resolved terahertz spectroscopy of dye-sensitized solar cells Transport in organic solar-cell materials studied by time-resolved terahertz spectroscopy Time-resolved terahertz spectroscopy technique is a specific case of optical pump-probe methods. It uses ultrashort optical pulses for excitation of a sample. Subsequent photo-induced changes of sample properties are probed by a short pulse of terahertz radiation (1 THz = 1012 Hz). Since the energy of the probing photons is low, this method is mainly sensitive to intraband excitations. In other words, time-resolved terahertz spectroscopy is a non-contact method for investigation of photo-initiated charge transport with a sub-picosecond temporal resolution. Generation and detection of terahertz pulses A common way to generate pulses of terahertz radiation is by means of optical rectification in non-linear crystals. When an intense ultrashort laser pulse hits such a crystal, its rapidly oscillating electromagnetic field is rectified, giving rise to a terahertz emission. Crystals of <110>-oriented ZnTe (zinc telluride) are the most common: when a 1 mm thick crystal is irradiated with 800 nm laser pulses of 100 fs duration, it emits broadband terahertz pulses covering a spectrum from 0 up to ~2.5 THz (Fig. 2). April 15, 2019 Refresher Course

A common way to generate pulses of terahertz radiation is by means of optical rectification in non-linear crystals. When an intense ultrashort laser pulse hits such a crystal, its rapidly oscillating electromagnetic field is rectified, giving rise to a terahertz emission. Crystals of 〈110〉-oriented ZnTe (zinc telluride) are the most common: when a 1 mm thick crystal is irradiated with 800 nm laser pulses of 100 fs duration, it emits broadband terahertz pulses covering a spectrum from 0 up to ~2.5 THz 2ps 0.1ps 100 fs Rectification of AC 800 nm = 374.741 THz 100 fs = 0.1 ps April 15, 2019 Refresher Course

April 15, 2019 Refresher Course